306 Stonsy— Of Atmospheres upon Planets and Satellites. 
2°38 kilometres* per second would escape, and, accordingly, the Moon is unable 
to retain any gas, the molecules of which can occasionally reach this speed at the 
highest temperature that prevails on the surface of the Moon. 
Shortly after, a second communication was made to the Royal Dublin Society, 
at one of its evening scientific meetings, based on the supposition that the Moon 
would have had an atmosphere consisting of the same gases as those of the Earth’s 
atmosphere, were it not for the drifting away of the molecules. It was shown 
that if the molecules of these gases can escape from the Moon, it necessarily 
follows that the Earth is incompetent to imprison free hydrogen; and this was 
offered as explaining the fact that, though hydrogen is being supplied in small 
quantities to the Earth’s atmosphere by submarine volcanoes and in other ways, it 
has not, even after the lapse of geological ages, accumulated in the atmosphere to 
any sensible extent. This communication was followed at intervals by others, 
in which the investigation was extended to other bodies in the Solar system, in 
which an endeavour was made to trace what becomes of the molecules that filter 
away from these several bodies, and in which it was suggested that the gap in 
the series of terrestrial elements between hydrogen and lithium may be accounted 
for by the intermediate elements [except helium] having escaped from the Earth 
at a remote time, when the Earth was hot. 
In one of the earlier of these communications, it was pointed out that it is 
probable that no water can remain on Mars—a probability which is now raised to 
a certainty by the recent discovery, that helium (with a molecular mass twice 
that of hydrogen) is being constantly supplied in small quantities to the Earth’s 
atmosphere by hot springs, and probably in other ways, and that nevertheless 
there is no sensible accumulation of it in the Earth’s atmosphere after the infil- 
tration has been going on for cosmical ages of time. In the absence of water, 
carbon dioxide was suggested as, with some probability, the substance that pro- 
duces the polar snows upon Mars. Moreover, on the Earth, snow, rain, and cloud 
are produced by the lightest constituent of our atmosphere ; but if the atmosphere 
of Mars consist of nitrogen and carbon dioxide, snow, frost, and fog on that 
planet are being produced by the heaviest constituent. An attempt was made to 
follow out the consequences of this state of things, and to refer to it those recurring 
appearances upon Mars which, though very imperfectly seen owing to the great 
distance from which we observe them, have been (perhaps too definitely) mapped 
and described under the name of canals. 
* It is very desirable that the names of metric measures should be made English words, and 
pronounced as such. Thus kilometre, hektometre, and dekametre should be pronounced with the accent 
on the second syllable as in thermometer, barometer, &c. This would have the further useful effect 
of better distinguishing these names from decimetre, centimetre, and millimetre, which have accents on 
the first and third syllables. 
